Silverblue doesn’t solve the same problems as Nix, or Ansible for that matter. I built my own in the past and it was non-trivial - although the CI process could pair quite nicely with Ansible. IMHO the primary advantage of Silverblue is that updates are a download, with practically zero work to do after the download has completed (this is a very big deal for RPM-based systems because an update boot can take a long time).
As for Ansible vs Nix, try switching from one program to another across all your machines. It’s doable but not fun. Now try switching back across all your machines. Nix makes your system equal a configuration, it does not add configuration.